1. Field of the Invention
The present invention relates to a three-dimensional ferroelectric capacitor that is suitable for being applied to a non-volatile semiconductor memory, for example, FeRAM (Ferroelectric Random Access Memory) and a method for manufacturing thereof as well as a semiconductor memory device using the three-dimensional ferroelectric capacitor.
2. Description of the Related Art
A ferroelectric type non-volatile semiconductor memory is composed of a dielectric capacitor section and selective transistor section, and its memory function is to make use of a spontaneous electric polarization phenomenon inherent in a ferroelectric material of the dielectric capacitor section to store information by matching the two stable states of the spontaneous polarization to [1] or [2]. This spontaneous polarization can invert its direction according to an electric field from the outside to change [1] or [2] information, and since the spontaneous polarization remains intact even after the electric field from the outside is removed, it functions as memory to store information even after a power supply is suspended. This ferroelectric-type non-volatile semiconductor memory has been put into practical use because of its characteristics such as high-speed information rewriting, low power consumption, high integration, and an enormously large number of times in information rewriting ability.
In addressing the miniaturization of cell areas due to the demand for a large capacity of semiconductor memory of late, use of ferroelectric materials as a dielectric film, such as SrTIO3 with high dielectric constants has been considered. However, the electric characteristics of these ferroelectric materials have been the cause of characteristic deterioration influenced by stress generated in an inter-layer dielectric film formed on the capacitor, compression stress occurring in a substrate, and tensile stress.
As a countermeasure, there are documents in which a three-dimensional semiconductor device is described to improve the structure of a semiconductor memory (for example, refer to Patent document 1). The semiconductor device described in patent document 1 has on a semiconductor substrate a dielectric capacitor including a column-structured first electrode, a dielectric film to cover the first electrode, and a second electrode to cover the first electrode and dielectric film, and is structured such that a non-electric conductor (low dielectric constant layer) is provided between the first electrode and dielectric film; and the semiconductor device operates effectively as a capacitor by sequentially laminating the dielectric film and second electrode on the side wall of the column-structured first electrode.
In addition, by providing the low dielectric constant layer between the first electrode and dielectric film, an electric field at the upper part of the lower electrode can be alleviated, so that an electric field in the dielectric capacitor can be focused on an approximately vertical direction to the sidewall of the lower electrode. Consequently, it becomes possible to prevent the electric field from locally focusing on the edge part of the lower electrode and, as a result, the reliability of the semiconductor memory is improved.
In the above-mentioned patent document 1, there is described a three-dimensional structure semiconductor memory in which the first electrode has a column-structure and the dielectric film and second electrode are laminated to cover the first electrode. In addition, even in the case where the dielectric film used in the dielectric capacitor uses a high dielectric film or ferroelectric film consisting of oxide or the like, in the document a semiconductor device is capable of restraining the characteristics deterioration of the dielectric film as occurs within the surface of the semiconductor substrate due to internal stress and an increase in a leak power current caused by the electric field focusing on the edge part of the lower electrode.
In the patent document 1 with respect to the three-dimensional structure semiconductor device, however, there is shown no intention or description of replenishing or increasing the declining amount of polarization in the ferroelectric film which may occur accompanied by miniaturization of the ferroelectric capacitor.
In addition, there is a description with respect to ferroelectric type non-volatile semiconductor memory cells of the planar type and stack type, in which a ferroelectric type non-volatile semiconductor memory cell is composed of a ferroelectric capacitor section and selective transistor element, and the ferroelectric capacitor section is composed of, for example, a lower electrode, upper electrode and ferroelectric layer held between these electrodes (for example, refer to Patent document 2).
[Patent document 1]
Japanese Laid-open Patent Application 2002-198495 (page 4 paragraphs [0011] to [0013], page 7 paragraph [0045])
[Patent document 2]
Japanese Laid-open Patent Application 2002-57297 (page 8 paragraph [0058] to page 9 paragraph [0065], page 9 paragraph [0068])
Conventionally, in order to improve the function of the ferroelectric capacitor (the amount of the electric charge of spontaneous polarization) of the ferroelectric type non-volatile semiconductor memory cell, it is important to form a film of the ferroelectric, in which crystallinity is excellent and the direction of crystallization (orientation) is aligned in a fixed direction. Of late, device elements tend to be miniaturized. As a result, capacity including spontaneous polarization per one element of the ferroelectric crystalline film that covers the lower electrode constituting the ferroelectric capacitor is reduced, so that the amount of the electric charge of spontaneous polarization of the ferroelectric capacitor tends to decrease.
The signal amounts of the ferroelectric capacitor of the ferroelectric type non-volatile semiconductor memory cell depend on the amount of polarization of the ferroelectric (SBT, PZT, BLT or the like) used. There are two measures to compensate for a decrease in the signal amounts due to miniaturization of 0.18 micrometer or less. One is to develop a technology for manufacturing a thin film crystal capable of forming homogeneous and microscopic grains in a microscopic capacitor while controlling the orientation of the ferroelectric thin film crystal. The other is to form a three-dimensional structure so as to expand the surface area of the capacitor in order to enlarge the limited area thereof.
The former measure is, however, strongly dependant on the property of respective ferroelectrics, so that it is difficult to control the orientation and to realize the miniaturization of grains as designed.
Therefore, the latter measure, in which the three-dimensional structure is formed without changing the material property of the respective ferroelectrics to expand the surface area so that the decrease in the amount of polarization due to miniaturization is compensated, is considered efficient.
As for the three-dimensional structure to expand the surface area of the ferroelectric capacitor, it is considered that a lower electrode and ferroelectric layer are laminated on a base structure that is etched to be processed into a rectangular solid to form a rectangular ferroelectric capacitor.
FIG. 1 is a conceptual diagram of a ferroelectric capacitor in which a three-dimensional structure of a rectangular solid is provided. The direction of crystallization of the upper surface (arrow mark A) and that of the side wall surface (arrow mark B) of the ferroelectric capacitor that is composed of a lower electrode 2 formed on an inter-layer dielectric layer 1 which is formed on a semiconductor substrate (not shown in the figure) and a ferroelectric crystalline film 3 formed on the lower electrode 2 are different in the direction of crystallization, that is, in orientation.
In addition, since the upper surface and sidewall surface each having a different orientation of crystallization collide with each other at a rectangular-shaped edge part 4, it is difficult to obtain continuous and homogeneous crystal, and a void tends to easily occur at this edge part, resulting in a crystallization defect. As mentioned so far, the ferroelectric crystalline film 3 having a three-dimensional structure is efficiently used in the case where the amount of the electric charge of the spontaneous polarization is increased by enlarging capacity, that is, expanding the surface area of the ferroelectric layer. However, there is a defect in the rectangular solid structure shown in FIG. 1.
The present invention was made in view of the above-mentioned circumstances, and aims at providing a three-dimensional ferroelectric capacitor used in FeRAM as is capable of compensating for a decrease in the amount of polarization due to miniaturization of devices by having the directions (orientation) of crystallization of a ferroelectric crystalline film aligned in an isotropic direction so that the surface area of a crystalline film is expanded, and a method for manufacturing thereof as well as a semiconductor memory device using the three-dimensional ferroelectric capacitor.